Gas combustors as described in the prior art are usually of the can or annular type designed to accommodate liquid and/or gaseous oxidant and fuel feeds. Generally, can combustors are advantageous in that they have better maintainability features. Efficient cooling of the can combustion chamber is essential in order to minimize disabling of the combustor liner or other structural components which would normally be subject to severe thermal stress. Accordingly, excess amounts of cooling air oxidant are necessarily used to moderate thermal effects within the combustion chamber and to cool the exhaust gases. In contrast to the high heating value fuels, e.g. natural gas, low BTU gas fuels, such as coal gas, require nearly stoichiometric fuel to air to obtain desired high temperatures (2600.degree.-3000.degree. F.), leaving little air for cooling purposes.
To negotiate critical cooling requirements, combustors of various designs are proposed in the art. Generally, the coolant air is fed to the combustor at predetermined points to achieve one or more of a number of objectives including, for example, cooling of the exhaust gases, provision of an air blanket or film immediately adjacent the combustor liner for heat and flame protection purposes, direction and control of the flame developed in the combustion zone to limit its size and configuration, etc. In other embodiments, air or fuel is introduced into air annular space defined by the liner and outer wall at a point downstream and passed forwardly to the fuel feed end of the combustion chamber, portions of the air being directed to the inner combustion zone at one or more intermediate points along its path. See for example, U.S. Pat. Nos. 3,077,073; 4,211,073 and 4,236,378.
Apparatus of the general type described in the referenced patents, however, are designed either for use with the high heating value fuels and use excess air over combustion requirements for cooling purposes. Moreover, the cooling effects attainable are often limited as a matter of design and inherent thermodynamic relationships and particularly since liner cooling is largely dependent on the available amount of air. Accordingly, problems associated with overall power plant cycle efficiency, combustor temperature, liner useful life and turbine blade and vane cooling are not necessarily avoided or effectively mitigated and particularly in the case of can type combustors.
Thus, a primary object of this invention is to provide a combustion chamber of the can type beneficially adapted for use with low BTU fuels wherein the foregoing and related problems are eliminated or at least substantially mitigated.
Another object is to provide such an apparatus wherein significant cooling effects are achieved despite relatively small quantities of available cooling air.
Still another object of the present invention is to provide such an apparatus wherein effective increase in overall power plant cycle and combustion efficiency, combustion liner useful life and efficiency of blade and vane cooling, are achieved.
A further object of the present invention is to provide such an apparatus enabling the use of higher combustor temperatures approaching the stoichiometric maximum for the particular low BTU gas fuel selected, thereby providing increased power output.
Other features, objects and advantages will become apparent upon reference to the succeeding detailed description of the invention and to the drawings illustrating the embodiments thereof.